This invention relates to methods for optically characterizing materials and signals carried therein.
Semiconductor and ferroelectric structures, such as DRAMS and CMOS circuits, are common in microelectronic, optoelectric, and photonic devices. Such structures often include multiple materials and layers, which may be separately processed, e.g., by chemical vapor deposition, ion-doping, and annealing, and patterned, e.g., by microlithography and ion-etching. Once fabricated, such structures often carry signals during use, e.g., charge carriers. Characterizing such structures and signal paths therein is important for developing new devices and manufacturing methods, and for assessing the quality of fabrication lines mass-producing existing devices.
Material structures can be characterized by local physical properties, such as stiffness, complex refractive index, and density. Waves propagating in the material structure can be indicative of such properties. For example, acoustic waves propagating within the material structures can be indicative of local stiffness variations.
The invention feature methods for characterizing a material, or signals therein, by imaging polariton waves propagating within the material. The methods and systems involve generating a polariton wave in the sample, allowing the polariton wave to propagate, and optically generating an image of the polariton wave. The image characterizes the propagation of the polariton wave, e.g., its spatially resolved direction, amplitude, and phase, at a time during or subsequent to the generation of the polariton. Imaging the polariton wave at multiple time intervals during and after its generation provides a series of images characterizing the time-dependent spatial propagation of the polariton wave. Since the phase velocity, attenuation, and direction of the polariton waves are sensitive to physical properties of the material, such as complex refractive index or dielectric constant, the polariton images are indicative of local and time-dependent variations in such properties. For example, the images can be indicative of material inhomogeneities including crystalline defects, domain wall boundaries, patterned structures, and other features fabricated deliberately or otherwise present that may scatter the polariton waves. Also, the images may indicate the presence of transient or dc electrical signals in the material since such signals may affect polariton propagation. Furthermore, in some embodiments, the material may be a device that uses polariton waves as signal carriers. In such embodiments the images characterize the polariton signal path, which may be important in diagnosing the efficacy of the device.
In general, in one aspect, the invention features a method for characterizing a polariton wave within a material. The method includes generating a polariton wave in the material and imaging the polariton wave with optical radiation to produce a spatially-resolved image of portions of the optical radiation affected by the polariton wave. The optical radiation may have a central wavelength in the range of about 300 nm to 2.5 microns.
In general, in another aspect, the invention features a method for characterizing polariton propagation within a material. The method includes: generating a polariton wave at a first spatial location in the material; waiting for a time interval sufficient to allow the polariton wave to propagate to additional spatial locations in the material; and optically imaging the polariton wave at the additional spatial locations. The polariton wave may have an electromagnetic frequency within the range of, e.g., about 300 GHz to 20 THz.
The optically imaging may include directing optical radiation to the additional spatial locations and generating a spatially-resolved image of portions of the optical radiation affected by the polariton. Also, the method may further include repeating the waiting and imaging steps for additional time intervals; and generating a spatially-resolved image of the polariton wave for each of the time intervals based on each of the imaging steps. Furthermore, the method may include identifying inhomogeneities in the material based on the images or detecting electrical signals within the material based on the images.
The polariton wave may be generated a number of ways. For example, it may be generated by converting fast electrical signals adjacent the first spatial location into the polariton or it may be generated optically, e.g., by crossing a pair of optical excitation beams on the material to form an optical excitation grating pattern at the first spatial location. Optical pulses having a durations shorter than 1 ps may be used to optically generate the polariton wave.
The optical imaging may be performed a number of ways. For example, it may be based on diffraction, polarization rotation, or spectral filtering of optical probe radiation that is transmitted through, or reflected from, the material. The material may be, e.g., a semiconductor or a ferroelectric.
Furthermore, the optical radiation directed to the additional spatial locations may have a size greater than or equal to about 1 mm. Also, the optical radiation directed to the additional spatial locations may have a size that overlaps the first spatial location.
In general, in a further aspect, the invention features a method for characterizing a polariton wave propagating within a waveguide. The method includes: introducing the polariton wave into a first location of the waveguide; waiting for a time interval sufficient to allow the polariton wave to propagate to additional locations within the waveguide; and optically imaging the polariton wave at the additional spatial locations.
The waveguide may formed in a photonic crystal. Also, the method may further include repeating the waiting and imaging steps for additional time intervals and generating a spatially-resolved image of the polariton wave for each of the time intervals based on each of the imaging steps.
The methods described above may have a number of advantages. For example, materials can be characterized based on polariton images. Furthermore, since each image provides information about multiple spatial regions of the material, the method provides a rapidly-acquired and intuitive xe2x80x9cpicturexe2x80x9d of the material response to polariton generation. Also, the methods and systems described above may be used as diagnostics for devices having complex material structures or structures used to carry signals.
Other features, aspects, and advantages will be apparent from the following detailed description, and from the claims.